I always thought of it this way:
First came Punk (Sex Pistols, The Clash, The Ramones, Siouxise and the Banshees, The Jam)...
then came New Wave, which was a result/reaction to punk (Elvis Costello, Talking Heads, Blondie, B-52's, Pretenders)...
then came everything else that was inspired by either or both Punk and New Wave, with so many varying styles that people just lumped them into New Wave. Styles such as:
Post-Punk (Wire, Magazine, Gang of Four, Echo and the Bunnymen, The Cure, Public Image Ltd., Joy Division, Siouxsie and the Banshees, Simple Minds, Cocteau Twins)
Synth-pop (Depeche Mode, The Human League, Orchestral Manoeuvres in the Dark, Tubeway Army/Gary Numan, Soft Cell)
New Romantics (Duran Duran, Spandau Ballet, Adam and the Ants, Visage, Culture Club).
Contrary to belief, there is a very strict boundary and huge difference between Synth-Pop and New Romantics. Synth-Pop was all about musical aesthetics and artistic exploration, with synths and keyboards obviously taking the forefront.
New Romantics on the other hand, did not require synths as a prerequisite, some New Romantics just happened to use synths. If you compare the sounds of Duran Duran with Adam and the Ants and Culture Club, all three bands sound entirely different. What held these bands to the term New Romantics was more of a style/visual aesthetic, with elaborate costumes, cosmetics, and avant-garde fashion... to the point where it was about escapism, hence being "Romantic." Of course, it had nothing to do with being "romantic" in a sense of mushy kind of love.
If you take a look at early Depeche Mode and OMD promo photos, you will notice they had no fashion sense whatsoever (Dave Gahan in plaid shirts! Andy McCluskey in woolen jumpers/sweaters!), hence they were NOT New Romantics. Pre-co-ed Human League and Gary Numan used costumes as an artistic expression, not fashion/escapism. Perhaps co-ed Human League can be described as New Romantics, but their fashion sense was originally branched out from the graphic design of their record sleeves. Depeche Mode later used leather and S&M stuff to coordinate with the music aesthetic of their subject matter, not as a "New Romantic" fashion thing. Same with Gary Numan with his various aesthetic changes.
OK, I'm done!